Green phosphorescent organic light emitting diodes with simple structure to realize an extremely low operating voltage

Relatively simple phosphorescent OLED with extremely low operating voltage (∼3.1 V) was devised. To eliminate some components of OLED device, the biased charge injection design was required. To confine the recombination zone toward the anode side, relatively large hole injection barrier (∼0.7 eV) and small electron injection barrier (<0.2 eV) were applied. To realize extremely low operating voltage, we utilized materials with relatively high mobility. Two different emitting layers containing hole transporting material (TAPC) mixed with two different electron transporting materials (TpPyPB and TmPyPB) showed slightly different device behavior. Two layer device with the highest efficiency was obtainable from the biased injection design (TAPC:TpPyPB system), while the biased injection was inefficient in case of TAPC:TmPyPB system.